It depends a lot on how the pop's powers are used and how skillful the player is. Let's compare pops.
Bombastic: A round of bombs on expert might run you into a powerdown or cause the bricks to drop once, but that's about it. If you're hugging the line, his attack can spell instant death, however, since the bricks are likely to kill you in the time it takes to catch all 4 bombs. A bomb spam in either situation can be pretty damaging to your position if you have to magnet the ball back.
Ioni: One round of flowers can keep you on you're feet unless you're highly experienced. Two or more rounds, and the ball will usually have to be either magneted or played up the side. Spamming with Ioni (either over and over again or after an attack wears off) is an underrated technique, in my opinion, particuarly against an opponent who isn't too sharp.
Ducky: Deceptively damaging! 1 duck won't cause you too much harm throughout a round, nor will 2 or 3. But every time the Ducky player has time to charge up his power, your situation on the board worsens little by little. Those little ducks can unexpectedly deflect a ball behind your paddle, or away from a crucial brick that's nearing the line of destiny. And those little buggers are tough to beat!
Mini-T: His attack can be very annoying against average or beginning players, but it's little more than a small nuscience once you've played against it enough. Medium-powered, overall, and highly dependent on the skill level of it's opponent. As everyone improves, I could see Mini-T as becoming weaker than the other pops.
Mr. Man: Those little bricklayers are pretty easy to take out in most cases, but if you get spammed with them, and they take paths at opposite ends of the playing field, they can be pesty. They'll almost always lay down a few bricks, which is very useful against brick players who like to clear the board, but Mr. Man's attack is, on the whole, a little weak unless you're hugging the line. Then he becomes a pretty large problem.
Tex: I haven't seen many Tex players out there. One attack rarely does anything, but it can give the Tex player a lucky win if his opponent is in danger. Two, three, or four attacks get sweaty, though, and in this case you are in a good position to miss the ball, unless you can anticipate its path and get there before your paddle is paralyzed. On the whole, I'd say Tex is one of the weakest pops. I hope someone proves me wrong one of these days, though.
Zap: Zap's sparks are kind of like 3 simultaneous ducks that happen to be constantly exploding. If the sparks are on the edge of the playing field, or if the ball is above them, they can actually aid Zap's opponent (much like the rubber Duckies), but they can cause quite a bit of frustration otherwise. In constant succession or in short burts, Zap's attack is annoying and effective.
Trying to rate the special attacks makes my head explode, so I won't do it. All I can say is that we'll all get a better idea when pop-pop is out for a little while longer.